Circadian oscillations arise both as an intrinsic cellular function and also through the interaction of cellular timekeeping neurons in neural circuits via peptidergic signaling both in mammals and in Drosophila. While it is clear that intact signaling via a neuropeptide called pigment dispersing factor (PDF) is required for coherent circadian rhythms in locomotor activity of Drosophila and that electrical activity in the pacemaker clock neurons can strongly effect the ticking of the cellular neuronal clock and vice versa, the mechanisms by which PDF signaling modulates these cellular properties are unknown. The long-term objective of this research proposal is to investigate the effects of neuropeptide signaling on the cellular clock and electrical activity of different subset of neurons within the circadian neural circuit by transgenically expressing a membrane-tethered form of PDF in different subsets of clock neurons. Preliminary studies have shown that driving membrane-tethered PDF in PDF-secreting neurons results in arrhythmicity of locomotor activity. On the other hand, constitutive PDFR activation by membrane-tethered PDF in other groups of neurons in the circadian neural circuit leads to either acceleration or deceleration of activity rhythms. In light of these data, a hypothesis emerges that peptidergic signaling through PDF and PDFR differentially affects the cellular oscillation and electrical activity of different subsets of clock neurons in the Drosophila brain. The proposed aims set out to test this hypothesis by 1) determining the effects of constitutive PDF signaling on circadian mRNA and protein expression of different subsets of clock neurons and 2) examining the effects of PDF signaling on the electrical excitability of different subsets of clock neurons. Since circadian rhythms are known to control many physiological processes, it is not surprising that perturbation of the circadian system has been linked to disease states, such as Familial Advanced Sleep Phase Syndrome and cancer. By deepening our understanding of how the circadian neural circuit is constituted and how it interacts with the cellular oscillations of individual clock neurons, a better understanding of circadian-related diseases and, ultimately, better treatments for these ailments will result.